Focusing device based on bonded plate structures

ABSTRACT

The invention provides a low mass and size thermal focusing device for a gas phase analytical device. The device has two or more plates bonded together. The device also has at least one channel with an entrance and exit is formed within allowing ingress and egress of a gas stream. The device also has a temperature reduction module in thermal contact with at least one plate of the device. Thermal focusing devices with additional plates and/or channels are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The technical field is gas phase analytical instrumentation, and inparticular, a focusing device for gas chromatography.

2. Description of Related Art

Gas chromatography is an analytical technique that separates compoundsvia gas-phase physicochemical processes. Samples comprised of mixturesof compounds are introduced into chromatographic system (sampleintroduction), vaporized (various means) if not gaseous already, movedby an inert gas stream (carrier gas) into and through a separationcolumn or columns. Sample components separate from each other when theytravel through the column at different speeds due to selectiveinteraction with the column and its coating or packing (the stationaryphase). Components eluting from the column are then detected by anappropriate detector.

Performance in gas chromatography is often reduced due to spreading ofsolute bands wider than their theoretically optimal widths. The optimalwidths are defined by well understood relationships of column (or columnpacking) dimensions, carrier gas type (e.g., He, H2, N2) and flow rate,stationary phase type and thickness, and temperature program rates.Sample introduction devices (inlets) that transfer sample into theanalytical column dictate the initial bandwidth of the sample. If sampleintroduction is slow, such as with splitless injection modes, headspaceanalysis, thermal desorption, etc., then some sort of focusing techniqueis required to narrow the input bandwidth sufficiently to be appropriatefor the given analytical system being used for separation. As solutebands move through the column, they naturally spread further. The degreeof spreading in the column is well modeled by known relationships. Insome applications, it is advantageous to narrow the width of solutebands eluting from one column as it passes to another column, adetector, or other zone. Having a narrow initial bandwidth in thesubsequent zone will often enhance chromatographic performance.

Various means for correcting for band spreading, that has occurred inone section, prior to release into another section have been developed.Methods for correcting band spreading or narrowing the bands are oftenreferred to as focusing. Current focusing methods, however, all havedisadvantages and limitations.

Focusing is typically accomplished through some combination of thermalfocusing and solvent focusing. The migration of solutes slowsapproximately 2-fold for every 25° C. decrease in temperature. So, insimple terms, thermal focusing works based on the principle that solutestend to “stick” to the stationary phase if temperatures aresignificantly lower than their elution temperatures. Temperatures atleast 50° C. less than the elution temperatures is typicallyrecommended, however, the lower the temperature, the less mobile thesolute and the more effective the focusing.

Solvent focusing relies on re-condensation of evaporated solvent in thehead of the column. This requires that the condensation zone be belowthe boiling point of the solvent. The lower the temperature below theboiling point of the solvent, the faster the condensation process andmore confined the solvent (and therefore solute) zone. The condensedsolvent acts to re-dissolve solutes that were evaporated in thechromatographic inlet. Evaporation expands the volume of a liquidseveral hundred times as it goes into the gaseous state. Re-condensationand re-dissolution reverse that effect, thereby reducing the volumeseveral hundred times as the vapors enter the cooled zone.

Thermal focusing or “cold trapping” in GC is known. However there arelimitations to the currently practiced trapping techniques. In onecommon implementation, the entire oven area of the instrument is cooled.This effectively turns the entire contents of the oven into a trap. Thisis useful for focusing broad bands that are caused by slow sampleintroduction (sample introduction device being outside the oven).However this implementation can not be easily be applied to focusingseparated bands eluting from one column prior to passing them to another(multidimensional chromatography) and require large amounts of coolingand heating because the whole oven area and contents are temperaturecycled.

Another common implementation is to direct cryogen (or another coolinggas) on a small section or column in the oven using a jet. The ovenstays at whatever higher temperature it is set at, while the smallsection of column experiences a lower local temperature. Although moreapplicable to multidimensional chromatography, this approach usuallyconsumes a large amount of coolant and at the same time requires morepower from the oven heater to compensate for the cooling of the cryogenbeing added to the oven.

Another approach is to use electro-thermal devices in contact with ashort section of column in the oven. This suffers from high failures andsuch devices have limited temperature cycling range (especially when ina heated oven environment).

SUMMARY OF THE INVENTION

The invention provides a low mass and size thermal focusing device for agas phase analytical device. The device has two or more plates bondedtogether. The device also has at least one channel with an entrance andexit is formed within allowing ingress and egress of a gas stream. Thedevice also has a temperature reduction module in thermal contact withat least one plate of the device.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail in the following descriptionof preferred embodiments with reference to the following figures.

FIG. 1 is a diagram of the features of the present invention.

FIG. 2 is a side view of one embodiment of the present inventionutilizing two plates.

FIG. 3 is a side view of an additional two plate embodiment of thepresent invention.

FIG. 4 is a side view of an additional two plate embodiment of thepresent invention.

FIG. 5 is a side view of an additional two plate embodiment of thepresent invention.

FIG. 6 is a side view of a three plate embodiment of the presentinvention.

FIG. 7 is a top view of a plate with material removed to form a slot.

FIG. 8 is a perspective view of a three plate embodiment of the presentinvention.

FIG. 9 is a perspective view of another three plate embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A focusing device for a gas phase analytical device is disclosed. Thebasic design of the diffusion bonded plate cryogenic device isillustrated in FIG. 1. FIG. 1 show an isolated inert sample path 5embedded between two diffusion bonded plates 10 and 20 with cryogeniczones above and/or below. Within the basic design there is greatflexibility of the physical configuration of the plates, sample path(s)dimensions (length and width of channel), and cryogen expansion zone(s).Specific embodiments illustrating the flexibility of the presentinvention are disclosed below. These embodiments are only examples anddo not limit the scope of the invention.

In one embodiment, sample paths 5 are created by etched depressions onplates 10 that are then sandwiched and sealed with other plates 20 inwhich channels have been etched, or alternatively to flat plates 20.FIG. 2 shows an exploded side view of two plates 10 and 20 of a focusingdevice of the present invention. As shown, the plates 10 and 20 are notyet sealed. The first plate 10 has a first surface 12. A channel 14 isetched into the first surface 12. The channel 14 begins at one end andextends in a continuous path to an exit, preferably on the opposite end(not shown) of the plate 10. The channels and features are added to aface of a stainless steel plate or variety of substrates typically usinga variant of the photolithography methods used in the chem-milling ofprinted circuit boards or printing plates used in offset presstechnology. Use of a substrate such as stainless steel has inherentadvantages relating to ruggedness, flexibility in integrating severaldifferent functions within one device, ease of manufacturing, inertness,compatibility with several common and proprietary forms of deactivation,and lifetime.

The second plate 20 has a second surface 22 that is flat and hassubstantially the same dimensions as the first surface 12. The firstplate 10 is fixed to the second plate 20 by sealing the first surface 12to the second surface 22. Bonding is accomplished by various weldingtechniques, for example, diffusion bonding, in the case of stainlesssteel substrates. Other materials, e.g. Ceramics, silicon, polymers,etc, can be bonded by various welding techniques. Once sealed, a samplepath is created with an inlet and outlet to the sample path that permitsingress and egress of a gas stream into the device. In one embodiment,chromatographic columns are connected to the inlet and the outlet. Thedepth and/or width of the channel 14 can be varied to create a largerbored sample pathway. Additionally, the length of the channel canincreased by etching for example a serpentine or zigzag pattern, asopposed to a straighter channel, to create a longer sample path.

Preferably, the sample flow path is inert for highest impact on thedesired application space. Inertness of the sample flow path can beachieved through various deactivation means. One way, for example, is touse plates made out of inert material. Another possible method forachieving inertness is to apply a thin coating of an inert substance onsurfaces in contact with sample. Preferred thin coating substancesinclude silica (e.g., SilicoSteel®), Sulfinert™, titanium nitride, andgold plating, although many other suitable materials could be used.Alternatively, or in combination with other techniques, coatings ofinert substance can be applied to improve focusing and increaseinertness. Preferred coating substances include common stationary phasesand common GC packing materials (e.g., porous polymers, coated packings,etc.). Any one of these methods for achieving inertness can be usedalone or in combination for greater effectiveness. In yet anotherembodiment, a pathway of the device serves as a housing for a length ofchromatographic column that can be inserted and then later removed andreplaced with a substitute length of column. Larger bore channels andeven larger channel free enclosures within bonded plates can be formedto serve as housing for a trapping column.

Key to effective focusing is lowering of the temperature of the device.The device in FIG. 2. has in thermal contact with the first plate 10 anelectro-thermal device 60, for example a Peltier, for cooling the samplepath for focusing and optionally heating it for desorption. Othermethods and means for both heating and cooling are discussed in moredetail below. This can be accomplished in a number of ways with thepresent invention. The nature of the design concept generally describedin FIG. 1 accommodates optimization of the design for many differentcoolant approaches. Plate dimensions, the number of layers of plates,the pathway(s) for coolant and restrictions necessary for efficientexpansion of cryogens, etc. can all be accommodated within the designconcepts.

FIGS. 3–5 illustrate additional embodiments utilizing two plates bondedtogether, similar to FIG. 2. In each embodiment two paths are createdwhen the plates are bonded, one pathway for sample and a second forcoolant. The difference between the embodiments disclosed in FIGS. 3–5and FIG. 2 is the location of the etching to create the channels thatwill form the sample and/or coolant paths. FIG. 3 shows the side view oftwo plates 10 and 20. A first channel 14 is etched into the firstsurface 12 of the first plate 10. The channel begins (or ends) at theside 11 of the first plate 10, and as shown in FIG. 2, extends in acontinuous path to an exit in a different side, preferably at theopposite end. The exit and entrance of the channel can be in any side,and even on the same side in some embodiments. In FIG. 3 a secondchannel 24 is etched into the second surface 22 of the second plate 20.The channel begins (or ends) at the side 21 of the second plate 20. Likethe first channel 14, the second channel extends in a continuous path toan exit. When the first plate 10 is bonded and sealed to the secondplate 20 in the direction indicated by the arrows, two paths arecreated. The channels 14 and 24 are positioned so that the channels 14and 24 do not intersect at any point in the device.

FIG. 4 shows another two plate embodiment, that when assembled is also atwo pathway device. In this embodiment the first channel 14 ispositioned on the first surface 12 of the first plate 10. A secondchannel 16 that does not intersect with the first channel 14 is alsopositioned on the first surface 12 of the first plate 10. The secondplate 20 is a solid plate of substantially the same length and width ofthe first plate 10. The second surface 22 of the second plate 20 isbonded to the first surface 12 of the first plate 10 in the direction ofthe arrows. The resulting device has two paths in the place of thechannels.

FIG. 5 shows yet another two plate embodiment, that when assembled is atwo pathway device. The first plate 10 and second plate 20 havesubstantially the same length and width. In this embodiment, like FIG.4, a first and second channel 14 and 16 are positioned on the firstsurface 12 of the first plate 10. The second plate 20 has a secondsurface 22. A third channel 24 and fourth channel 26 are etched in thesecond surface (22) at locations corresponding to the first channel 14and second channel 16 respectively. When bonded two paths are formed. Inthis way paths with larger diameters can be formed from similarlydimensioned plates to the embodiment in FIG. 4.

A two path device, such as those describe in FIGS. 3–5 can accommodatean incorporated cooling means. In a two path device, one path may beused as a sample path, and the second path can be used for theintroduction of a coolant to focus the sample. The coolant pathpreferably runs parallel to the sample path for the most efficientcooling. Many types of coolant gases and sources may be usedeffectively.

For most effective cooling, cryogens are used as the coolant. Typicalcryogens include evaporation of liquid CO₂ and liquid N₂, and aresuitable for use with the devices of the present invention. Differentcryogens have different design requirements for most efficient use andare familiar to those skilled in the art. The different restrictionsnecessary for expansion of the two gases can easily be integrated intothe plate design.

An alternative coolant is compressed air blown through coolant channelof the focusing device to lower its temperature. This is a more costefficient method but has thermal limitations. It is most effective ifthe temperature of the compressed air is significantly lower than thetemperature of the device (e.g., the GC oven temperature is >150° C.hotter than the air), but this is not a requirement.

An alternate alternative approach for cooling the device is vortexcooling. In terms of effectiveness of cooling, vortex cooling issomewhere between the use of cryogens and the use of compressed air. Invortex cooling, a vortex tube creates a vortex from compressed air andthen separates it into hot and cold airstreams. On entering a vortextube, the compressed air flow passes through a vortex generationchamber, which starts the air stream rotating. The air stream exitingthe chamber rotates at very high speeds (e.g., speeds up to 1,000,000rpm) as it is forced along the inner walls of the tube towards a controlvalve. At the control valve, a small portion of the air exits through aneedle valve as hot exhaust. The remaining air is forced back throughthe center of the incoming air stream at a slower speed, where theslower moving return air gives up heat energy to the faster movingincoming air. The cooled return air then flows through the center of thevortex generation chamber and exits through the cold air exhaust port.The cold air can then be flowed through the path in the device. Vortexcooling typically reduces incoming temperatures 35–45° C., making itquite suitable for use with the invention in certain applications.

The present invention lends itself to a number of cooling options inaddition to those described above, such as, for example, Peltier coolingdescribed in FIG. 1, the use of refrigerants and others.

Once focused, solutes must be desorbed effectively from the device. Thisis accomplished through heating. The present invention accommodatesseveral approaches to heating. The simplest is to merely stop the flowof coolant and let the device equilibrate with its ambient (e.g., GCoven) temperature. The invention works well with this approach due toits low thermal mass and uniform heat conduction but may not be idealfor every application.

Faster and higher temperature heating of the device can be accomplishedby direct heating. When the device is designed with electricallyconductive materials, an electrical current can be driven across thedevice, which operates to heat up the device. Appropriate control andtemperature sensing circuits are typically used when controlled heatingis desired.

In another embodiment indirect heating can be used. This is especiallyappropriate when non-conductive materials or unsuitable materials areused to construct the device. For example heat traces can be integratedinto the device, or heating cartridges can be attached, as well as otherindirect heating methods. Typically, appropriate temperature feedbackand control devices would be used with indirect heating as well.

Other heating approaches are also possible, and suitable, with thedisclosed device. For one example, if cold effluent from a vortexgenerator is used to cool the device, the hot effluent flow could alsobe used to heat the device. In an analogous manner, a Peltier could bereversed to supply heat for desorption. In another embodiment, ifrefrigerant is used to cool the device, the process could be reversed tosupply heat in a manner similar to heat pumps, with the condensing coilseither attached to or designed within the device.

Just as the invention can accommodate many types of cooling and heatingmethods, the present invention has great flexibility with theconstruction of the device. For example, two plate embodiments have beendescribed. The present invention can be configured using more than twoplates. For example, FIG. 6 illustrates one three plate embodiment.

In FIG. 6, a first plate 10, a second plate 20, and a third plate 30 arebonded together to form the device. The first plate 10 has a firstsurface 12. The second plate 20 has two surfaces 22 and 27. A firstchannel 24 is etched into the first surface 22 and a second channel 28is etched in the second surface 27. The third plate 30 has a firstsurface 32 and side 31. The first plate 10 and the third plate 30 arebonded to the second plate in the direction of the arrows forming twopathways on either surface of the second plate 20.

FIG. 6 is only one illustration of a three plate embodiment; otherembodiments utilizing three or more plates is contemplated by thepresent invention. Many different configurations of plates and channelplacements are possible to achieve the desired combination of channellength and diameter necessary for effective focusing. For example, athree plate embodiment can have more than one path between each plate.Longer paths can be formed in a such a three plate device by providingthe appropriate feed through holes from one pathway and connections to aseparate pathway in the same device.

In yet another embodiment sample paths can be achieved by completelyremoving material (creating slots) on the plate, and then sandwichingthe plate between two other solid plates to create a pathway in thedevice. In this approach, alternating slotted plates with solid platescan yield larger paths. FIG. 7 illustrates a plate 70 with a slot 74created by the removal of material from the plate 70. A center point 75is positioned to receive a feed through hole.

FIG. 8 illustrates the plate 70 and the two outside plates 80 and 90.Plates 80 and 90 are bonded in the direction of the arrows and create asample path. A feed through hole at 85 is lined up with the center 75 ofthe plate 70. Plate 90 is solid with no breaks or holes. Additionalplates (not shown) can be added as desired to create additional pathwaysif desired.

The disclosed invention can be integrated with other fluidic devices.For example, a natural combination is to couple a Deans switch formultidimensional GC with the cryofocusing device of the presentinvention. Additional combinations are possible including effluentsplitters, combiner of multiple flows (focusing effluent from severalstreams into a single place).

Having described preferred embodiments of a novel focusing device forgas chromatography (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made inlight of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments of the inventiondisclosed which are within the scope of the invention as defined by theappended claims.

Having thus described the invention with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1. A thermal focusing device comprising a plurality of plates and atemperature reduction means, wherein: the plates are bonded together andat least one channel with an entrance and exit is formed within; and thetemperature reduction means cools a first channel of the at least onechannel so as to trap a gas sample within the first channel, thetemperature reduction means being in thermal contact with at least oneplate.
 2. A thermal focusing device according to claim 1, wherein theplurality of plates includes: a first plate having a first surface witha first groove formed therein; and a second plate having a secondsurface, wherein the first and second surfaces are bonded together sothat the first groove forms a first channel of the at least one channel.3. A thermal focusing device according to claim 2, wherein: the secondsurface has a second groove formed therein; and the first and secondgrooves face each other forming the first channel.
 4. A thermal focusingdevice according to claim 3, wherein: the first surface has a thirdgroove formed therein; the second surface has a fourth groove formedtherein; and the third groove faces the fourth groove so as to form asecond channel when the surfaces are bonded together.
 5. A thermalfocusing device comprising a plurality of plates and a means fortemperature reduction in thermal contact with at least one plate,wherein: a first plate has a first surface; a second plate has a secondand a third surface; the second surface is bonded to the first surfaceso that a first groove in the second surface forms a first channel; thethird surface has a second groove; and a third plate has a fourthsurface that is bonded to the third surface so that the second grooveforms a second channel.
 6. A thermal focusing device according to claim1, wherein one or more of the channels is coated with an inertsubstance.
 7. A thermal focusing device according to claim 1, whereinone of said channels contains a stationary phase substance.
 8. A thermalfocusing device according to claim 1, wherein one of said channels iscoated with an inert substance and contains one of a liquid stationaryphase substance and a solid stationary phase substance.
 9. A thermalfocusing device according to claim 1, wherein the temperature reductionmeans comprises an electro-thermal device.
 10. A thermal focusing deviceaccording to claim 1, wherein the temperature reduction means includesan enclosed channel of the at least one channel for conveying one of acryogen and a coolant through at least one of said plates.
 11. A thermalfocusing device according to claim 10, wherein: the enclosed channel isfor conveying the cryogen through the at least one of said plates; andthe enclosed channel includes a cryogen expansion zone.
 12. A thermalfocusing device according to claim 10 further comprising heating meansfor heating and desorbing the trapped sample, the heating means being inthermal contact with at least one plate.
 13. A thermal focusing deviceaccording to claim 12, wherein the heating is means includes at leastone of a heating cartridge and a heat trace integrally formed with theplurality of plates.
 14. A thermal focusing device according to claim12, wherein: the at least one plate is formed of an electricallyconductive material; and the heating means includes an electric currentsource configured to pass an electric current through the at least oneplate.
 15. A thermal focusing device according to claim 12, wherein: thetemperature reduction means and the heating is means are comprised of avortex tube; temperature reduction is accomplished by a cold airstreamfrom the vortex tube; and heating is accomplished by a hot airstreamfrom the vortex tube.
 16. A thermal focusing device comprising: a middleplate with a first surface and a second surface and with materialcompletely removed between the first and second surfaces of the middleplate to form a continuous pathway with a beginning and an end; twosubstantially solid endplates bonded to the middle plate, wherein oneendplate is bonded to the first surface and the other endplate is bondedto the second surface and wherein a channel formed within forms thecontinuous pathway; and temperature reduction means in thermal contactwith at least one plate to trap a gas sample in the continuous pathway.17. The thermal focusing device according to claim 16 wherein: thebeginning of the pathway is in the center of the middle plate; the endof the pathway is at an outer edge of the middle plate; and one endplate has an aperture aligned with the beginning of the pathway.
 18. Athermal focusing device comprising: a first plate having a firstsurface; a second plate having a second surface that is bonded to thefirst surface, wherein at least one of the first surface and the secondsurface has a first groove formed therein so that the bonded first andsecond plates form a corresponding first channel; a third plate having athird surface that is bonded to a fourth surface of the second plate,wherein at least one of the third surface and the fourth surface has asecond groove formed therein so that the second and third plates bondedat the third and fourth surfaces form a corresponding second channel;means for cooling at least one of the said plates to trap a gas samplein the first channel; and heating means in thermal contact with at leastone plate.
 19. A thermal focusing device according to claim 18, whereinthe second plate has at least one aperture therethrough arranged topermit communication between the first and second channels forming onecontinuous channel.
 20. A thermal focusing device according to claim 19wherein the channel is coated with an inert substance.
 21. A thermalfocusing device according to claim 2, further comprising a third plate,wherein: the second plate has a third surface with a second grooveformed therein; the third plate has a fourth surface; and the third andfourth surfaces are bonded together so that so that the second grooveforms a second channel of the at least one channel.
 22. A thermalfocusing device according to claim 3, further comprising a third plate,wherein: the second plate has a third surface with a third groove formedtherein; the third plate has a fourth surface; and the third and fourthsurfaces are bonded together so that so that the second groove forms asecond channel of the at least one channel.
 23. A thermal focusingdevice according to claim 2, further comprising a third plate, wherein:the second plate has a third surface; the third plate has a fourthsurface with a second groove formed therein; and the third and fourthsurfaces are bonded together so that so that the third groove forms asecond channel of the at least one channel.
 24. A thermal focusingdevice according to claim 3, further comprising a third plate, wherein:the second plate has a third surface; the third plate has a fourthsurface with a third groove formed therein; and the third and fourthsurfaces are bonded together so that so that the third groove forms asecond channel of the at least one channel.